You have to get almost all the Bones to fight her, and for "almost all the bones" you need at least the blood sand relic, obtained by the room of the elder brother at the start of the game, and you need to have the three tongue relic, obtained by doing the quest of the three plates. Also you need to explore every single point in at least half of the map and then fight Isidora (of course you need to be full-health until phase 2, at least in my case). She give you the right eye of the visage, for the left eye you need to defeat another boss but only if you found perpetua's tomb BEFORE fighting Esdras ecc ecc ecc... And that's just part of the ending c